Fabricating integrated circuits with less coupling capacitance and higher density is a major goal in the manufacturing of very large scale integrated (VLSI) circuits. As device geometries are reduced to the micrometer and submicrometer range, further developments in silicon gate metal-oxide semiconductor (MOS) processing methods are needed.
In the prior art, CMOS structures having a common gate for both n and p channel devices have been fabricated using bulk silicon for the p channel device (lower transistor) and a laser recrystallized silicon film for the n channel device (upper transistor), (see e.g., J. F. Gibbons and K. F. Lee, "One-Gate Wide CMOS Inverter on Laser-Recrystallized Polysilicon", IEEE Electron Device Letters, Vol. EDL-1, No. 6, June, 1980). The Gibbons and Lee structure is vertically built and obtains high packing density by means of a self aligned common gate structure. However, the CMOS process used to produce this common gate device results in a complete source and drain overlap of the gate for the upper transistor. This overlap contributes to large coupling capacitance, which is undesirable for high performance MOS devices. Reduction of the coupling capacitance results in high circuit speed and low power drain in the device. Thus, it is important to obtain a MOS process which minimizes coupling capacitance and obtains high packing density. In addition, it is desirable that such a process be compatible with standard VLSI processing.